Because it is incredibly difficult. It has been discussed at length a number of times and it always concluded that is a staggering amount of work or it could not be faithful enough. I myself have sculpted 3 doom style monster and while I think they look nice, I would not call them faithful.

For example when I first played doom and didn't know the monster names. I called the cacodemons "Man Eating Strawberries"
Edit. As a bonus what did you think the wolfenstein enemies were saying when you alerted them?

I mentioned how to take the sounds out in an earlier post. For the weapons, just erase the ones you don't want from the brutal weaponsection in the KEYCONF lump and move the rifle to slot 2. If you have need for it, you can PM me and I will tell you what to do with the weapon DECORATES. I also added an awesome secondary fire the the pistol, launcher, and pump shotgun.
Messing with some of these things caused a few minor bugs but the above changes caused none. When I finish with this tedious knife sprite sheet I am working on I will iron out the bugs and share. Thanks for letting me know about that extension. I will check it out.

After some more time with it, I have basically turned it into v20 with the increased gore and enhancement script. Gone with smg, auto shotgun, grenade launcher, pistol melee, launcher scope, ridiculous chaingun fire rate, foot step sounds, marine chatter, much more like this.

This is going to sound stupid but I did not know about the shareware version of doom. I though whole game was knee deep in the dead. i played it one or twice and played doom 2 since then..... Needless to say, I have been distracted and playing doom for about two days because of the "bonus" episodes.